For many years there has been an increasing interest in wireless communications using cellular radio communications networks (cellular networks hereinafter). The cellular networks were originally based on analogue radio technologies and used foremost for fixed or mobile radio telephony services. Modern cellular networks are digital and are used for an increased range of services including various data services, such as messaging, downloading of music, video and other files, email communications, Internet access etc. The cellular industry is constantly seeking ways to provide new or improved services. For example, with the introduction of EDGE (Enhanced Data rates for GSM Evolution) technology for GSM (Global System for Mobile communications) and the HSPA (High Speed Packet Access) for WCDMA (Wideband Code Division Multiple Access), data communication using cellular networks has reached new heights. Development of IMS (IP Multimedia Subsystem) provides IP (Internet Protocol) based communications in a variety of modes—including voice, text, pictures and video, or any combination of these—in a highly personalised and secure way. IMS may, for example, be used to provide PoC (Push-to-talk over Cellular).
The cellular network has a design that is based on a number of radio nodes, often referred to as base stations, base transceiver stations, node Bs, radio access points or similar. Hereinafter, for the sake of simplicity, the term base station will be used as a generic expression covering any such radio node. The base stations provide radio coverage in one or more geographical regions, commonly referred to as cells. A user located in one of the cells can access the cellular network with a so-called user equipment (UE), i.e. equipment having necessary radio communication capabilities for communicating with the cellular network via the base stations. For example, the user equipment can be a mobile radio terminal (e.g. a mobile phone) for services such as speech and selected data communications, a personal computer having associated radio communications equipment, a mobile communicator for services such as speech and extensive data communications, a fixed radio telephone etc. The cellular network normally includes a core network linked directly of indirectly to the base stations for providing basic communication services, such as switching and transport, as well as the above-mentioned higher level services. Mobility is normally supported by the cellular network, i.e. the cellular network allows the user to move within the network while using communication services. This is usually accomplished by means of handovers between base stations. Handovers can of course also be performed between cellular networks, in order to support so-called roaming.
One example of a so-called third generation cellular network is the Universal Mobile Telecommunications System (UMTS), which is standardised by an undertaking known as the Third Generation Partnership Project (3GPP). The radio access network in UMTS is abbreviated UTRAN (UMTS Terrestrial Radio Access Network). Key components of the UTRAN are the base stations (Node B) and the Radio Network Controllers (RNC). Communications between the user equipments and the UTRAN is performed over a standardised radio interface, most commonly based on WCDMA (Wideband Code Division Multiple Access). The UTRAN additionally provides connectivity between the core network and the user equipments. The RNC is a governing control node controlling a number of base stations connected to the RNC. The 3GPP specified interface between the RNC and each associated base station is referred to as Iub. NBAP (Node B Application Protocol) is control plane protocol, which is designed to manage the logical resources of the base stations. The NBAP forms the basis for radio network control signalling between the RNC and the base stations and essentially handles all procedures between these two nodes. The NBAP protocol may be subdivided into NBAP common (C-NEAP) and NBAP dedicated (D-NBAP). The C-NBAP controls overall Node-B functionality. C-NBAP procedures are procedures that request initiation of a base station Communication Context for a specific UE in the base station or are not related to a specific UE. C-NBAP procedures also incorporate logical O&M procedures. Consequently, C-NBAP is used for controlling the base station itself, and for setting up dedicated connections towards the UEs. This implies that the common procedures are of a more essential nature. If they are unavailable, it is in principle impossible for RNC to control the base station. D-NBAP procedures are procedures that are related to a specific base station Communication Context in the base station. This base station Communication Context is identified by a Node-B Communication Context identity. Consequently, D-NBAP is used for communication with individual UEs. Each UE is related to a “communication context”, thus is it possible to control a plurality of UEs simultaneously. The dedicated procedures are considered less critical, since they only relate to phone call(s).
NBAP control plane data is carried on transport bearers over the Iub interface between the RNC and the base stations. The 3GPP (3GPP TS 25.432) specifies various transport technology options for NBAP signalling over the Iub interface. The traditional transport option is ATM (Asynchronous Transfer Mode), but the 3GPP standard also supports the more modern IP (Internet Protocol) based transport option. Many operators are therefore currently migrating from the ATM based transport option to the IP based transport option. One known way to effect this migration involves the following. All traffic on the base station is stopped in a clean way, i.e. without dropping any calls. The ATM based NBAP connection is thereafter terminated, e.g. closed or cut. The base station is adapted to exhibit a timeout behaviour in response to the termination. That is, when the base station detects an interruption on the NBAP connection, the base station waits for a predetermined time (normally about ten seconds) and, if the NEAP connection is still interrupted, the base station takes down all its cells. This will drop all calls and stop all base station operation within a few seconds. A completely new NEAP connection is established using the new IP based transport option. This procedure can in principle be performed within a few minutes, but nevertheless requires base station down time, and is therefore generally performed at night. The technique requires extra planning and scheduling. This leads to increased costs and lost revenues for the operator, and to bad working hours for staff. Dropped calls may also negatively influence a reputation of the operator.
Consequently, an object of the present invention is to overcome or a least mitigate at least one of the above-indicated difficulties.